(1) Field of the Invention
The invention relates to the manufacture of highly dense integrated circuits, and more particularly to the reduction of electrostatic charge buildup at a Metal Oxide Semiconductor (MOS) device during manufacturing processes.
(2) Description of the Related Art
In the manufacture of highly dense integrated circuits using MOS technology, unwanted electrical charges may build up at the device gate oxide during plasma processing. The charge accumulates on floating polysilicon and metal layers electrically connected to the gate oxide. Conductive surfaces act as "antennas", amplifying this charging effect and leading to trapped charges at the gate oxide. These trapped charges can cause yield loss and reliability failures.
Workers in the field have verified this problem experimentally. Shone et al in "Gate Oxide Charging and Its Elimination for Metal Antenna Capacitor and Transistor in VLSI CMOS Double Layer Metal Technology" (published in "Symposium on VLSI Technology, pp. 73-74 in June, 1988) verified the antenna effect experimentally. They found that the double metal layer technologies worsened the effect by the ratio of the antenna area to the gate oxide area. While the worst degradation of gate oxide occurred during oxynitride deposition, other plasma processing is also believed to lead to trapped charges at the gate oxide. One suggested remedy was the attachment of a substrate diode to the metal antenna.
Others have attempted to resolve these problems. In U.S. Pat. No. 5,350,710 (Hong, et al.), an antifuse device is formed between the metal antenna and the device to be protected, in order to isolate the device during plasma processing. Later in the manufacture of the circuit, a short-circuit is formed at the antifuse by applying a sufficiently high voltage across the antifuse location, to form the final connection between the metal and device.
While the above solutions satisfactorily avoid the antenna effect during plasma processing after formation of the metal layer, they do not address the problem of plasma processing prior to metal formation, for example between formation of polysilicon layer(s) and metal formation.